Sealing glass paste method

ABSTRACT

A method of controlling the rheology of a sealing glass paste which comprises incorporating a fine inorganic powder in the paste as a mill addition to impart a steep rheology curve to the paste so that the paste resists flow while at rest, but flows readily when shear stress is applied, the amount of inorganic powder incorporated being not over about 10% by weight.

This application claims the benefit of U.S. Provisional Application No.60/016,477, express mailed Apr. 29, 1996, entitled SEALING GLASS PASTEMETHOD, by Edward J. Fewkes, Margaret E. Greene, Robert Morena andFrances M. Smith.

FIELD OF THE INVENTION

Method of controlling the rheology of a sealing glass paste used inproducing a fusion seal.

BACKGROUND OF THE INVENTION

Glasses that melt at relatively low temperatures have long been used tojoin metal, glass and ceramic components by a fusion seal. Initially,such seals were vitreous in nature. Lead glasses, particularly lead zincborates and borosilicates, were commonly used.

The sealing art was revolutionized by discovery that such glasses couldbe crystallized in a controlled manner by thermal treatment. Thiscombined the virtues of a low melting glass with a strong seal in thecrystalline state.

More recently, non-lead glasses, composed essentially of tin, zinc, andphosphorous oxides, have been proposed as a substitute for thetraditional lead zinc borate glasses. U.S. Pat. No. 5,246,890 (Aitken etal.) discloses examples of such phosphate glasses.

In forming a seal with such sealing glasses, it has become customary touse the glass in a particle form known as glass frit. The glass frit ismixed with a vehicle and binder to form a suspension, or paste. Thismixture is applied to a sealing surface, for example, by extrusion.

Numerous organic vehicles and binders have been proposed. A combinationof nitrocellulose and amyl acetate is commonly used in commercialcathode ray tube production. Recently, environmental concerns aboutvolatile organic compounds (VOCs) have given rise to a proposal forsubstitution of an aqueous solution of a cellulosic polymer.

In preparing a sealing glass paste, serious consideration must be givento the rheological characteristics of the paste, that is, its flow orviscosity characteristics. The paste needs to be relatively fluid forapplication purposes. However, both before and after application, whenthe paste is at rest, it should be relatively stiff or viscous.

Prior to application, there may be a substantial delay betweenpreparation of the paste and its use. This may be due to transportationor storage. If the paste is not sufficiently viscous, settling orseparation of components may occur.

Frequently, a paste is applied by being extruded as a ribbon of the formrequired for sealing. It is desirable that the paste set quickly afterapplication so as to avoid any flow that would cause loss of theextruded shape.

In order to meet these contradictory requirements, it has been proposedto employ a paste having the characteristics of a reversible gel. Such apaste exhibits a high viscosity during storage and/or after application,but exhibits a much lower viscosity when placed under shear stress. Thisstress may be created in a mixer, or by pressure applied duringextrusion.

U.S. Pat. No. 4,260,406 (Corbett et al.) describes introducing a gellingagent into a paste to impart the characteristics of a reversible gel. Anumber of organic and inorganic gelling agents are disclosed as beinguseful. However, certain surfaceactive organic titanates, known astitanate coupling agents, are indicated to be preferred.

This known technique of controlling rheology is effective. However, itwould be desirable to provide even better control. It would also bedesirable to obtain such control without resort to the known gellingagents. In particular, it would be desirable to employ onlynon-volatile, non-combustible, inorganic additives.

It is a basic purpose of the present invention to provide a method ofrheology control in a sealing glass paste that achieves these ends.Another purpose is to provide a method of control that utilizes anadditive having the desired characteristics. A further purpose is toprovide a method that permits an extended delay between preparation of apaste and its use without settling, or other undesirable separation,occurring. A still further purpose is to provide a method of rheologycontrol that permits relatively easy application of the paste to asealing surface.

SUMMARY OF THE INVENTION

The invention resides in a method of controlling the rheology of asealing glass paste which comprises incorporating a fine, inorganicpowder that is compatible in the paste as a mill addition to impart asteep rheology curve to the paste so that the paste resists flow whileit is at rest, but flows readily when shear stress is applied, theamount of fine, inorganic powder incorporated being not over about 10%by weight of the sealing paste.

The invention further resides in a method of preparing a sealing glasspaste for use in producing a fusion seal, which comprises controllingthe rheology of the paste by incorporating a fine, inorganic powder inthe paste as a mill addition to impart a steep rheology curve to thepaste so that the paste resists flow while at rest, but flows readilywhen shear stress is applied, the amount of inorganic powderincorporated being not over about 10% by weight.

PRIOR ART

Prior literature of possible interest is listed in an accompanyingdocument.

BRIEF DESCRIPTION OF THE DRAWING

In an accompanying drawing,

FIG. 1 is a graphical representation of the invention as applied to anon-lead sealing glass, and

FIG. 2 is a graphical representation of the invention as applied to alead sealing glass.

FIG. 3 is a graphical representation of the invention showing itsgeneral applicability.

DESCRIPTION OF THE INVENTION

The present invention arose from efforts to provide a simple, buteffective, method of controlling the rheology of a sealing glass paste.Such pastes are widely used in producing fusion seals to join componentparts. These components may, for example, be parts of cathode ray tubes,electric lamp envelopes, and display screens. Present commercialpractice in preparing a sealing glass paste employs a sealing glass fritdispersed in a vehicle of nitrocellulose dissolved in amyl acetate.

A sealing glass paste is prepared by dispersing the glass, in the formof frit, in the vehicle. The frit may be dispersed by agitating inequipment, such as a Hobart double planetary mixer, or any of variousknown mixers, to produce a homogeneous mass. The ratio of glass frit tovehicle may vary widely, but the amount of glass generally exceeds theamount of vehicle. Typically, the glass frit, on a weight basis, may be80-90% of the mixture in the absence of mill additions.

As noted earlier, it is desirable to produce a sealing glass pastehaving the rheological characteristics of a reversible gel. Thisrequires the paste to be relatively stiff as prepared in order to avoidsettling of the heavier fit on standing. However, a stiff paste cannotbe easily dispensed onto a sealing surface. Therefore, the paste shouldchange to a relatively fluid state for application purposes. Inaccordance with the present invention, this results from shear stress,such as occurs in mixing, or during extrusion.

The Corbett et al. patent describes use of gelling agents, particularlyorganic titanates to achieve that end. Other modifiers have also beenproposed for the purpose.

The present invention achieves the rheological characteristics of areversible gel by adding a fine inorganic powder, with a high surfacereactivity, to the paste mixture as a mill addition. The fine powderaddition imparts a superior rheology to the paste whereby thedispensability of the paste is greatly facilitated. It accomplishes thiswithout the addition of either rheology modifiers or surfactants.

The effect on rheology has been studied using fine alumina powderbecause of the ready availability of that material commercially.However, as shown subsequently, the effect does not appear to be limitedto that material. Rather, it appears to be due to the fine particle sizeof a material, not the materials chemical constitution.

We believe that the improved rheology, observed with a fine inorganicpowder, is due to increased interaction with the polymer nitrocellulose.When the polymer is coiled at low shear, the particles clump togetherand the paste appears viscous. During high shear, the polymer extends,the particles separate, and the paste appears to be thinner. In anyevent, the effect is observed in the absence of added surfactants orrheology modifiers.

Our studies with alumina have shown that the unique rheological effectis achieved with a commercially available alumina having an averageparticle size of 0.5 microns. Coarser aluminas, in the range of 6-15microns average particle size, have proven relatively ineffective. Ingeneral, the alumina must have an average particle size below 3 microns,preferably less than 1 micron. Up to about 10% by weight of the finepowder may be added without unduly affecting paste consistency. However,we prefer to employ no more than about 1% in a blend.

The effect on rheology has been found to be independent of the sealingglass composition. Thus, it has been found to be effective in both thecommercially used lead glasses and in non-lead glasses, such as thephosphate disclosed in the Aitken et al. patent.

Lead borate, or lead borosilicate, glasses consist essentially of 70-80%PbO, 10-14% ZnO, 6-12% B₂ O₃, 0-3% SiO₂ and 0-3% BaO. In addition tobeing lead-free, the Aitken et al. glasses have compositions containing25-50 mole % P₂ O₅ and SnO and ZnO in amounts such that the mole ratioof SnO:ZnO is in the range of 1:1 to 5:1. They are of particularinterest for use in producing seals in cathode ray tube envelopesbecause of their relatively low tin oxide contents. For other purposes,higher ratios up to 10:1 or higher may be useful.

The Aitken et al. glass compositions may further contain up to 20 mole %modifying oxides including up to 5 mole % SiO₂, up to 20 mole % B₂ O₃,and up to 5 mole % Al₂ O₃. They may also contain one or morecrystallization promoters selected from 1 to 5 mole % zircon and/orzirconia and 1-15 mole % R₂ O. Additionally, the composition may includea seal adherence promoter selected from up to 5 mole % WO₃, up to 5 mole% MoO₃, up to 0.10 mole % Ag metal and mixtures.

The invention has been largely developed in conjunction with thecommonly used commercial nitrocellulose binder dissolved in amylacetate. However, considerable interest has developed recently withrespect to vehicles having no volatile organic compounds (VOCs) assolvents. We have found that the present rheological effect is obtainedequally in conjunction with such vehicles, including polymeric bindersand organic/aqueous systems.

TABLE I sets forth a series of non-lead sealing glass compositions, andsealing paste blends employing these non-lead glass frits. The glassfrits are blended with aluminas of varying particle size. The glasscompositions are given in mole %, while the blends are in weightpercent. The amount of alumina in each blend was maintained relativelyconstant to better compare the effect of particle size. The glass inblends 2-4 has a pyrophosphate stoichiometry, whereas that in blend 1 isa near orthophosphate. This illustrates the general applicability of theinvention to non-lead glasses.

                  TABLE I                                                         ______________________________________                                        Blend        1      2          3    4                                         ______________________________________                                        P.sub.2 O.sub.5                                                                            28.5   31.5       31.5 31.5                                        ZnO 6.4 14.6 14.6 14.6                                                        SnO 63.6 51.2 51.2 51.2                                                       B.sub.2 O.sub.3 1 1.5 1.5 1.5                                                 Al.sub.2 O.sub.3 0.5 0.7 0.7 0.7                                              WO.sub.3 0 0.5 0.5 0.5                                                        Base Frit 70 74 74 74                                                         Beta-eucryptite 10 0 0 0                                                      Alumina (0.5μ) 5 0 3 6                                                     Alumina (6-8μ) 0 6 3 0                                                     Zircon 15 20 20 20                                                          ______________________________________                                    

Sealing glass pastes were prepared from each of the four blends by theaddition of glass frit to an organic vehicle solution. The solution was1.2% nitrocellulose in amyl acetate, and the frit to vehicle ratio was5.8:1 on a weight basis. The pastes were mixed with a high speeddisperser set at 600 rpm. The pastes thus prepared were analyzed on aRheometrics RDA-II parallel plate analyzer. This analyzer is a parallelplate mechanism in which one of the plates is oscillated. A materialsample is placed between the plates, and viscosity is measured as theoscillation rate is increased.

Rheological curves obtained from measurements made on the four blendsdescribed in TABLE I are plotted on FIG. 1 of the drawing. FIG. 1 is agraphical representation of viscosity measurements made on the dynamicanalyzer. Viscosity is plotted in log poise on the vertical axis;oscillatory frequency is plotted on the horizontal axis inradians/second (rad/s). The rheological curves are identified by numberscorresponding to the blend numbers in TABLE I.

The increase in steepness of the rheological curves with increasingaddition of fine alumina is evident. This unique effect of fine alumina,as contrasted to coarser alumina additions, is also evident. Thus,curves 1 and 4 are markedly steeper than curve 2 wherein the blendcontained only coarse alumina. Curve 3, wherein the total aluminacontent was the same, but was split between coarse and fine particlesize, shows an intermediate effect. The closeness of curves 1 and 4indicate the independence of the viscosity effect from composition ofthe glass frit.

A steep rheology curve for a paste indicates that the paste will floweasily when exposed to an increase in shear. The steeper the curve, thegreater the flow in a material when shear is applied. Another desirablecharacteristic of a paste having a steep curve is that it is very thick(or viscous) when at rest. This is desirable because it prevents a pastefrom settling during storage, and from deforming after application to asealing surface. Thus, curves 1 and 4 are relatively steep in the lowshear region of about 1 rad/s.

TABLE II sets forth, in weight %, a similar series of sealing glasspaste blends in which alumina additions are made. Again, the aluminaparticle size is varied. The frit employed in these blends is alead-zinc-borosilicate having a composition on a weight basis of:

    ______________________________________                                                PbO   74.5                                                              ZnO 12.9                                                                      B.sub.2 O.sub.3 8.4                                                           SiO.sub.2 2.2                                                                 BaO 1.9                                                                       Al.sub.2 O.sub.3 0.03                                                       ______________________________________                                    

Blend 5 in TABLE II has a mixture of coarse aluminas added. Blend 7 hasthe same amount of alumina added, but in a single coarse particle size.Blend 6 has a corresponding addition of alumina, but in a fine particlesize. A sealing glass paste was prepared with each of the blends ofTABLE II. The procedure described above was employed, except that thefrit:vehicle ratio was approximately doubled to nearly 12:1. Rheologymeasurements were made with the oscillatory analyzer, also as describedabove.

FIG. 2 displays the rheology curves for the blends of TABLE II. Again,the curves are identified by numbers corresponding to the blend numbers.FIG. 2 is a graphic representation corresponding to FIG. 1, and havingmeasurements plotted thereon in the same manner.

It will be observed that essentially the same effect on viscosity isobserved with the lead-zinc-borosilicate frits of TABLE II that wasobserved with respect to the non-lead frits of TABLE I. Thus, curve 6,based on blend 6 with the fine alumina addition, is much steeper thancurves 5 and 7. These curves represent blends 5 and 7 which contain thesame amount of alumina as blend 6, but having a coarser particle size.

                  TABLE II                                                        ______________________________________                                        Blend          5           6     7                                            ______________________________________                                        Lead Base Frit 96          96    96                                             Alumina (0.5μ) 0 4 0                                                       Alumina (6-8μ) 2 0 4                                                       Alumina (4-6μ) 1 0 0                                                       Alumina (10-15μ) 1 0 0                                                   ______________________________________                                    

TABLE III sets forth, in weight percent, a further series of sealingglass paste blends. In this TABLE, blends 8 and 9 contain additions ofzirconia, while blends 10 and 11 contain comparable additions ofalumina. The frit employed had the tin-zinc-phosphate composition shownin blend 1 of TABLE I.

                  TABLE III                                                       ______________________________________                                        Blend          8     9          10  11                                        ______________________________________                                        Frit           70    70         70  70                                          Beta-Eucryptite 10 10 10 10                                                   Zircon 15 15 15 15                                                            Zirconia (0.5μ) 5 0 0 0                                                    Zirconia (6-8μ) 0 5 0 0                                                    Alumina (0.5μ) 0 0 5 0                                                     Alumina (6-8μ) 0 0 0 5                                                   ______________________________________                                    

It will be observed that blends 8 and 10 contain very fine particle size(0.5 μ) zirconia and alumina, respectively. Likewise, blends 9 and 11contain larger particle size (6-8μ) zirconia and alumina, respectively.

These blends were mixed in the manner described above to produce sealingglass pastes. Rheology measurements were made with the oscillatoryanalyzer as described above. FIG. 3 displays the rheology curves forblends 8-11 based on these measurements.

FIG. 3 is a graphic representation corresponding to FIGS. 1 and 2 withrheology measurements plotted thereon in like manner. The rheologycurves are identified by numbers corresponding to the blend numbers inTABLE III.

It will be observed that the rheological effects, shown by the fineparticle size versus the coarser particle size materials, are virtuallythe same with zirconia as with alumina. Thus, both oxides provide muchsteeper rheological curves (8 and 10), as well as higher low shearviscosities, when present in the very fine size. While other compatiblematerials are not readily available in the very fine particle size,there is every reason to believe they would demonstrate a similareffect.

We claim:
 1. A method of controlling the rheology of a sealing glasspaste which comprises blending a compatible, inorganic powder with asealing glass as a mill addition to impart a steep rheology curve to thepaste, so that the paste resists flow while at rest, but flows readilywhen shear stress is applied, the inorganic powder having a particlesize less than 6 microns and being present in the blend in an amount notover about 10 percent by weight.
 2. A method in accordance with claim 1which comprises blending an inorganic powder of alumina or zirconia withthe sealing glass.
 3. A method in accordance with claim 2 whichcomprises blending alumina with the sealing glass.
 4. A method inaccordance with claim 3 which comprises blending alumina with thesealing glass in an amount up to about 6 percent by weight of the glassplus alumina.
 5. A method in accordance with claim 3 which comprisesblending alumina having a particle size not over about 3 microns withthe sealing glass.
 6. A method in accordance with claim 5 wherein theparticle size of the alumina is under one micron.
 7. A method inaccordance with claim 1 which comprises blending the powder with asealing glass that is a non-lead, tin-zinc-phosphate glass frit.
 8. Amethod in accordance with claim 1 which comprises blending the powderwith a sealing glass that is a lead-zinc-borosilicate glass frit.
 9. Amethod of improving a sealing glass paste, for use in producing a fusionseal, which comprises controlling the rheology of the paste by blendingan inorganic powder in the paste as a mill addition to impart a steeprheology curve to the paste so that the paste resists flow while atrest, but flows readily when shear stress is applied, the amount ofinorganic powder blended being not over about 10 percent by weight andhaving a particle size of less than 6 microns.
 10. A method inaccordance with claim 9 which comprises controlling the rheology of thepaste by incorporating alumina or zirconia in the paste.